1. Field of the Invention
The present invention relates to a knock control system for an internal combustion engine, and more particularly to a knock control system for an internal combustion engine, in which an amount of ions produced by combustion in the internal combustion engine is detected, to thereby detect an occurrence of knocking in the internal combustion engine, and correct a combustion parameter of the internal combustion engine so that the knocking is suppressed.
2. Description of the Related Art
Generally, in an internal combustion engine, air and fuel (an air-fuel mixture) introduced into a combustion chamber in each cylinder is compressed by an upward movement of a piston, and in an explosion stroke, a high voltage is applied to a spark plug in the combustion chamber to burn the compressed air-fuel mixture with electrical sparks generated at the spark plug. Explosion energy produced at this time is taken out as a downward force of the piston, and is converted into a rotational output.
In the above-mentioned explosion stroke, when the combustion occurs inside the combustion chamber, molecules inside the combustion chamber dissociate (ionize). Therefore, immediately after the explosion stroke, if a high voltage is applied to an electrode for detecting an ionic current, which is arranged inside the combustion chamber, the charged ions are allowed to flow as the ionic currents.
Besides, it is known that the ionic current varies sensitively based on a combustion state in the combustion chamber. Therefore, the combustion state (occurrence of misfire or knocking) inside the cylinders can be distinguished by detecting the states of the ionic currents.
Therefore, in the conventional art, there has been proposed a system for detecting an occurrence of knocking in the internal combustion engine by detecting the state of the ionic current, such as described in, for example, Japanese Patent Application Laid-open No. Hei 10-9108. In the knock control system for an internal combustion engine described in Japanese Patent Application Laid-open No. Hei 10-9108, a frequency band corresponding to the knocking is extracted from the ionic current by means of a band-pass filter, to serve as a knock signal. A number of knock pulses obtained by comparing the knock signal with a predetermined level is then used to determine the occurrence of knock.
In a case where a noise, which has a frequency close to that of the knock and is capable of passing through the band-pass filter, is generated in the ionic current, a pulse that corresponds to the noise is generated. However, taking this phenomenon into consideration, the detected number of pulse is subjected to averaging, and a result obtained by subtracting the average number of pulses from the number of knock pulses is assumed as the number of pulses which actually corresponds to actual knocks. Then, an amount of retardation control is increased by an amount corresponding to the number of pulse.
Incidentally, when the knock detection is performed based on the ionic current, typically there is little difference in the knock detection sensitivity levels among the cylinders. Thus, proposals have been made of an ignition timing control for each cylinder, such as in the conventional devices described in the above-mentioned publications. However, there occurs a case in which differences in the noise levels among the cylinders are generated by differences among the cylinders caused by soiling of the spark plug, wearing of the spark plug electrode, and the like. This may cause over knock judgement intense or non-detection of the knock to occur at only one cylinder.
The above-mentioned conventional devices propose a measure of calculating a knock judgement threshold value for each cylinder. However, it is confirmed through experiments that there are cases where this countermeasure alone is insufficient. One reason for this resides in that even if a knock intensity judgement method, in which the judgement is made based on the ionic currents (i.e., the number of pulses obtained by comparing the signals that have passed through the band-pulse filter with the predetermined level, according to the conventional devices), is employed, occurring of the soiling of the spark plugs, wear of the spark plug electrodes, or the like does not stabilize the noise levels, which makes it difficult to learn the noise levels perfectly.
The present invention has been made to solve the above-mentioned problems with the conventional art, and therefore has as an object to provide a knock control system for an internal combustion engine, with which appropriate ignition timing control for each cylinder can be made, even in a case where differences in noise levels of knock detection signals generated from ionic currents, are generated for each cylinder, thereby being difficult to prevent erroneous detection by simply learning the noise levels.
According to the present invention, a knock control system for an internal combustion engine includes: knock detection means for evaluating an ionic current generated at a time of combustion of an air-fuel mixture in an internal combustion engine to output a knock detection signal; threshold value setting means for setting a threshold value for use in a knock judgement performed on the knock detection signal; and knock judgement means for performing the knock judgement based on the threshold value and on the knock detection signal. Also, the knock control system for an internal combustion engine includes: control parameter correction request amount setting means for setting a control parameter correction request amount that includes at least ignition timing, based on the knock judgement result; control parameter correction means for setting the control parameter correction amount based on the control parameter correction request amount to correct the control parameter; and variation correction means for performing a correction to reduce variation exhibited by a cylinder in a case where at least one of the knock detection result and the control parameter correction amount exhibits great variation with respect to those of other cylinders.